I'm confused.
FCOM:
The Flight Augmentation Computers have three main functions:
- Rudder trim
- Rudder travel limits
- Yaw damping inputs
- Alternate yaw
- Flight envelope and speed computations
- Wind shear detection
Can it be said that the FACs rely upon pitot tubes (three of them, but nothing else) to determine the aircraft's airspeed?
Can it be said that in the case of
double FAC failure that the aircraft will revert to alternate law -- with reduced protections?
Can it be said that the FACs might interpret (misinterpret) rapid airspeed fluctuations known to be associated with crossing shear layers / windshear, and implement overspeed protection
trimming the aircraft nose up?:
Flight crew operating manual(FCOM)and standard procedures
•Until mid 2013, there was only a descending aircraft over speed prevention checklist That checklist included disconnecting the autopilot and raising the aircraft nose.Use of that checklist in the cruise was inappropriate because it could result in a significant altitude exceedance at high speed and manually flying the aircraft at high speed and high altitude was not practiced often.
•Airbus published new flight crew operating manual (FCOM) procedures for overspeed prevention and recovery. Both checklists commence with AP (autopilot) :KEEP ON
. A newsletter distributed to company flight crew contained the new overspeed recovery FCOM but not the overspeed prevention FCOM.
•If the autopilot remained engaged during an aircraft overspeed flight envelope speed protection is provided in the relevant AFS modes, resulting in a nose up order within the limit of the autopilot authority to reduce or stop the airspeed increase
Can it be said that once a UAS event is triggered, the AP disconnects but A/THR remains active, the FDs remain active and the FAC / protections remain active (to the extent the FACs have reliable - or unreliable information?)
In the event of rapid transient true overspeed while crossing shear layers associated with thunderstorms, is it reliable to depend upon FACs and pre-programmed
overspeed protections? Taken a step further, can perceived overspeed protections affect the SS nose down / manual pitch trim down a pilot might apply?
After entering alternate law, due to UAS prompting dual FAC disagree / failure, at what point does the FAC drop itself out?
Automatic pitch trim stops trimming nose down momentarily at VMO or MMO. This provides a momentary pitch up when high speed stability is active, but automatic trimming will continue if over speed is continued by pilot. An aural "crickets" warning will be heard at VMO plus 4 knots or / MMO plus .006M.
High Speed Stability may or may not be available in alternate law depending on the type of flight control failure.
If a zoom climb/ambient air temperature change/violent updraft, etc., resulted in the aircraft being out of its envelope, in 'approach to stall' does the FAC drop itself out after it has sensed the aircraft approaching a perceived stall speed or does the Augmentation Computer still try to 'help':
The VSW is defined by the top of a red and black strip along the speed scale. It represents the speed corresponding to the stall warning,
as computed by the FACs.
When do the FAC / protections cease having any effect on pilot inputs / pilot directions?
Is there any way to turn them off -- if, say, you were already close max alt, nearing coffin corner/impending stall, yet encountering more turbulence/windshear and HAL wanted more nose up?
When the A320 encounters wind condition changes in the cruise similar to shear, the airspeed can increase quite quickly and this is not a rare event on the A320. During flights between Melbourne and Gold Coast/Brisbane airports, this windshear/wind speed change and potential overspeed situation is not uncommon as the aircraft transits the southern jetstream.
AIRBUS SAFETY LIBRARY
II.2 Defining Windshear
Windshear is defined as a sudden change of wind velocity and/or direction.
Windshear occurs in all directions, but for convenience, it is measured along vertical and horizontal axis, thus becoming vertical and horizontal windshear:
Vertical windshear:
−Variations of the horizontal wind component along the vertical axis, resulting in turbulence that may affect the aircraft airspeed when climbing or descending through the windshear layer
−Variations of the wind component of 20 kt per 1000 ft to 30 kt per 1000 ft are typical values, but a vertical windshear may reach up to 10 kt per 100 ft.
Windshear conditions usually are associated with the following weather situations:
•Jet streams
•Mountain waves
•Frontal surfaces
•Thunderstorms and convective clouds
•Microbursts.
Influence of Windshear on Aircraft Performance The flight performance is affected as:
• Headwind gust instantaneously increases the aircraft speed and thus tends to make the aircraft fly above intended path and/or accelerate
• A downdraft affects both the aircraft Angle-Of-Attack (AOA), that increases, and the aircraft path since it makes the aircraft sink
• Tailwind gust instantaneously decreases the aircraft speed and thus tends to make the aircraft fly below intended path and/or decelerate.
Windshears associated to jet streams, mountain waves and frontal surfaces usually occur at altitudes that do not present the same risk than microbursts, which occur closer to the ground.
I don't see thunderstorms there....
